Methods for reducing nitroaromatic compounds have received interest as the products from the partial or total reduction of those nitroaromatic compounds have found an expanding variety of uses. These uses include drug intermediates, antibiotics, pesticides, herbicides, radiosensitizers and explosives which may be produced with nitroaromatics as starting materials. As used in these applications, the nitroaromatic compounds are partially or totally reduced as part of the processing required for production of the final product.
Additionally, nitroaromatic compounds in many circumstances have been proven to create environmental or health hazards. For example, nitrobenzene has been shown to cause headaches, drowsiness, nausea, vomiting and methemoglobinemia with cyanosis. Nitrobenzene has also been shown to be toxic to rats with LD50 of 640 mg/kg.
The current stockpile of energetic materials requiring resource recovery or disposition (RRD) is 449,308 tons. Through 2001, over 1.2 million tons will pass through or reside in the RRD account (Joint Ordnance Commands Group; 1995). A totally different but significantly similar challenge exists in clean-up of the sites where soil and ground water are contaminated with TNT, RDX, HMX, and other nitro-based explosives. Hence, there is a need for characterizing the reactions of explosives with naturally occurring enzymes, and a cost-effective technology to degrade these contaminants.
To take advantage of the potential uses of partially and totally reduced nitroaromatic compounds, and to eliminate nitroaromatic compounds in circumstances where they pose environmental or health risks, a variety of processing schemes have been developed to bring about the partial or total reduction of these nitroaromatic compounds. Many such schemes involve the use of naturally occurring enzymes to catalyze the reduction. Such schemes are highly advantageous as the enzymes are often readily obtainable and their use as catalysts minimizes undesirable waste and byproducts. An example of the use of such enzymes is provided by Sommerville (Sommerville, C., Nishino, S. F., and Spain, J. C. (1995) J. Bacteriol., 177, 3837-3842), wherein it was demonstrated that nitrobenzene may be reduced to phenylhydroxylamine through the use of oxygen insensitive nitrobenzene reductase as a catalyst. Schemes such as that described in Sommerville are characterized by an inability to control the reduction using a simple inhibitor such as molecular oxygen and further, that they require the use of NADPH, an expensive reductant. However, it is often desirable that the reduction of the nitro groups not be allowed to progress to completion, as it is desirable to isolate or collect a partially reduced product.
U.S. Pat. No. 5,777,190 to Shah et al., is directed to a method for the controlled reduction of nitroaromatic compounds such as nitrobenzene and 2,4,6-trinitrotoluene (TNT) by enzymatic reaction with oxygen sensitive nitroreductase enzymes, such as ferredoxin NADP oxidoreductase. Through the addition of oxygen, the reduction of nitroaromatic compounds may be halted at the point at which a partially reduced product has been produced. Again, a disadvantage of this process is that it uses expensive cofactors like nicotinamide adenine dinucleotide phosphate (NADPH) for TNT reduction.
Hence, there remains a need for a cost-effective method of controlled reduction of nitroaromatic compounds.